Added blocking dequeue and peek methods

Added Dque.DequeueBlock() method that, when the queue is empty, blocks (doesn't return) until something is added to the queue. There is also a corresponding Dque.PeekBlock() method. Thank you, Thomas Kriechbaumer.

protect the queue directory by a lockfile

Thomas Kriechbaumer added a lock file to prohibit opening of the queue by different processes (or even just different threads) at the same time.